Vehicle drive force control apparatus

ABSTRACT

A vehicle driving force control apparatus is configured to provide a vehicle drive force control apparatus that can expand the operating region within which an electric motor can be driven in a stable manner. The output of an engine is delivered to the main drive wheels through a transmission and also to an electric generator. The output voltage of the electric generator is supplied to the electric motor and the output of the electric motor drives the subordinate drive wheels. When it is determined that the generator output voltage will not be sufficient to meet the requirement of the electric motor, the amount of acceleration slippage allowed at the left and front wheels is increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a vehicle drive force control apparatus for a vehicle having a wheel or set of wheels driven by a main drive source and an electric generator also driven by the main drive source. Preferably, an electric motor is driven with electric power generated by a generator that is driven by the main drive source. The present invention especially useful in an all wheel drive vehicle in which a pair of main drive wheels are driven by the main drive source, such as an internal combustion engine, and a pair of subordinate drive wheels are driven by the electric motor. Thus, the invention is particularly well-suited for a so-called battery-less four-wheel drive vehicle in which the engine drives the generator and the electric power from the generator is supplied to the electric motor.

2. Background Information

One example of a drive force control apparatus is disclosed in Japanese Laid-Open Patent Publication 2000-318473. The drive force control apparatus of this publication discloses a vehicle in which the front or main drive wheels are driven by an internal combustion engine, and the rear or subordinate drive wheels are driven by an electric motor. The electric motor is driven by electric power generated by an electric generator that is driven by the engine. When the drive force control apparatus disclosed in this publication is used, it is not necessarily mandatory to have a battery that supplies electric power to the electric motor. Furthermore, the device presented in this publication is configured to assist with acceleration from a stopped condition by driving the rear wheels with the electric motor only when the vehicle is starting into motion from a stop.

In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved vehicle drive force control apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.

SUMMARY OF THE INVENTION

The vehicle drive force control apparatus described in the above mentioned publication is configured such that the motor is driven only during a limited traveling state, i.e. only when the vehicle is starting into motion from a stopped condition. For example, the motor is driven only when the gear ratio of the transmission is in the first speed range.

Since the voltage supplied to the electric motor is affected by the rotational speed of the engine driving the electric generator, there is the possibility that a situation will occur in which the electric generator is temporarily unable to deliver the required voltage to the electric motor, e.g., when the transmission upshifts from first speed to second speed. When this kind of situation occurs, the motor torque driving the rear wheels will temporarily decrease.

The present invention was conceived in view of this shortcoming. One object of the present invention is to provide a vehicle drive force control apparatus that is capable of expanding the operating range in which the electric motor can be driven in a stable manner.

In order to resolve the aforementioned shortcoming, the present invention proposes a vehicle drive force control apparatus that is configured such that when it is determined that the generator output voltage required by the electric motor cannot be obtained, the amount of acceleration slippage permitted at the main drive wheel or main drive wheels is increased so as to curb the decline in generator output.

In view of the foregoing, a vehicle drive force control apparatus is provided that basically comprising a main drive source, an electric generator, an electric motor, a main drive source output suppressing section, and an acceleration slippage degree revising section. The main drive source is configured and arranged to drive a main drive wheel. The electric generator is configured and arranged to be driven by the main drive source. The electric motor is configured and arranged to drive a subordinate drive wheel from output voltage supplied from the electric generator. The main drive source output suppressing section is configured to suppress the output of the main drive source such that a degree of acceleration slippage of the main drive wheel is matched to a preset target slippage degree when the main drive wheel undergo acceleration slippage. The acceleration slippage degree revising section is configured to increase the target slippage degree upon estimating that at least one of a target generator output voltage will likely be insufficient to achieve a target motor output of the electric motor, and a gear ratio of a transmission has increased.

These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of this original disclosure:

FIG. 1 is a schematic block diagram of a vehicle equipped with a vehicle driving force control apparatus in accordance with preferred embodiments of the present invention;

FIG. 2 is a block diagram showing a control system configuration for the vehicle driving force control apparatus illustrated in FIG. 1 in accordance with the illustrated embodiments of the present invention;

FIG. 3 is a block diagram showing the 4WD controller for the vehicle driving force control apparatus illustrated in FIG. 1 in accordance with the illustrated preferred embodiments of the present invention;

FIG. 4 is a flow chart showing the processing sequence executed by the surplus torque computing section of the 4WD controller for the vehicle driving force control apparatus illustrated in FIG. 1 in accordance with the illustrated embodiments of the present invention;

FIG. 5 is a flow chart showing the processing sequence executed by the target torque control (limiting) section of the 4WD controller for the vehicle driving force control apparatus illustrated in FIG. 1 in accordance with the illustrated embodiments of the present invention;

FIG. 6 is a flowchart showing the processing executed by the surplus torque converting section in accordance with the first embodiment of the present invention;

FIG. 7 is a flowchart showing the processing executed by a TCS control section in accordance with the first embodiment of the present invention;

FIG. 8 is a simplified diagram plotting different target motor torques versus the vehicle speed and target engine rotational speed;

FIG. 9 is a simplified diagram plotting the target slippage ratio revision amount verses the engine rotational speed difference;

FIG. 10 is a simplified diagram plotting the target slippage ratio revision amount versus the engine rotational speed difference for each different gear ratio;

FIG. 11 is a flowchart showing the processing executed by the engine controller in accordance with the first embodiment based on the present invention;

FIG. 12 is a flowchart illustrating the processing executed by a TCS target revising section in accordance with a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

First Embodiment

Referring initially to FIGS. 1–11, a vehicle driving force control apparatus will now be explained in accordance with a first embodiment of the present invention. As seen in FIG. 1, a four wheel drive vehicle is diagrammatically illustrated that is equipped with the vehicle driving force control apparatus in accordance with the present invention. As shown in FIG. 1, the vehicle in accordance with this embodiment has left and right front wheels 1L and 1R that are driven by an internal combustion engine or main drive source 2, and left and right rear wheels 3L and 3R that are driven by an electric motor or subordinate drive source 4, which is preferably a direct current (DC) electric motor. Thus, the front wheels 1L and 1R serve as the main drive wheels, while the rear wheels 3L and 3R serve as the subordinate drive wheels. The output torque Te of the engine 2 is delivered to the left and right front wheels 1L and 1R after passing through a transmission 5 a and a differential gear 5 b. An endless drive belt 6 transfers power from the internal combustion engine 2 to an electric generator 7, which supplies electrical energy to the electric motor 4. A portion of the output torque Te of the engine 2 is delivered to the electric generator 7 by the endless drive belt 6. In other words, the generator 7 is rotated at a rotational speed Nh, which is obtained by multiplying the rotational speed Ne of the internal combustion engine 2 by the pulley ratio of the endless drive belt 6. Additionally, the rotational speed Nh of the generator 7 can be computed based on the rotational speed Ne of the internal combustion engine 2 and the pulley ratio of the endless drive belt 6.

The transmission 5 a is provided with a shift or gear position detecting device 5 c (gear ratio detecting device) configured to detect the current shift range or gear position of the transmission 5 a. The shift position detecting device 5 c sends a signal indicating the detected shift position to a 4WD controller 8. The transmission 5 a executes gear shifting in response to a shift command from a transmission control unit (not shown in the drawings). The transmission control unit holds tables or the like containing information describing the shift schedule of the transmission based on the vehicle speed and the accelerator position. When it determines that the vehicle will pass through a shift point based on the current vehicle speed and accelerator position, the transmission control unit issues a shift command to the transmission.

The generator 7 rotates at a rotational speed Nh that is equal to the product of the rotational speed Ne of the internal combustion engine 2 and the pulley ratio of the endless drive belt 6. The load (torque) 110 placed on the internal combustion engine 2 by the generator 7 due to the field current Ifh of the generator 7 is adjusted by the 4WD controller 8 to generate a voltage corresponding to the load torque. The voltage generated by the generator 7 can be supplied to the electric motor 4 through the electrical line 9. A junction box 10 is provided at an intermediate point in the electrical line 9 between the electric motor 4 and the generator 7. The drive shaft of the electric motor 4 can be connected to the rear wheels 3L and 3R via a reduction gear 11, a clutch 12 and a differential gear 13 in a conventional manner.

The internal combustion engine 2 has an air intake passage 14 (e.g., the intake manifold) that includes a main throttle valve 15 and a subordinate throttle valve 16. The throttle opening of the main throttle valve 15 is adjusted/controlled in accordance with the amount of depression of the accelerator pedal 17, which also constitutes or functions as an accelerator position detecting device or sensor, or a throttle opening instructing device or sensor. In order to adjust the throttle opening of the main throttle valve 15, the main throttle valve 15 is either mechanically linked to the depression amount of the accelerator pedal 17, or adjusted/controlled electrically by an engine controller 18 in accordance with the depression amount detection value from an accelerator sensor 29 that detects the depression amount of the accelerator pedal 17 or the degree of opening of the main throttle valve 15. The depression amount detection value from the accelerator sensor 29 is outputted as a control signal to the 4WD controller 8. The accelerator sensor 29 constitutes an acceleration or throttle instruction sensor. Thus, the phrase “accelerator position opening degree” as used herein refers to either a throttle opening amount of the main throttle valve 15 or a depression amount of the accelerator pedal 17 or similar accelerator device.

The subordinate throttle valve 16 uses a stepper motor 19 as an actuator for adjusting its throttle opening. Specifically, the throttle opening of the subordinate throttle valve 16 is adjusted/controlled by the rotational angle of the stepper motor 19, which corresponds to the step count. The rotational angle of the stepper motor 19 is adjusted/controlled by a drive signal from the motor controller 20. The subordinate throttle valve 16 is provided with a throttle sensor 19 a shown in FIG. 2. The step count of the stepper motor 19 is feedback-controlled based on the throttle opening detection value detected by this throttle sensor 19 a. The output torque of the internal combustion engine 2 can be controlled (reduced) independently of the driver's operation of the accelerator pedal 17 by adjusting the throttle opening of the subordinate throttle valve 16 so as to be smaller than the throttle opening of the main throttle valve 15.

The vehicle driving force control apparatus is also equipped with an engine rotational speed sensor 21 that detects the rotational speed Ne of the internal combustion engine 2. The engine rotational speed sensor 21 outputs control signals that are indicative of the engine rotational speed Ne to both the engine controller 18 and the 4WD controller 8.

A brake pedal 34 is provided that constitutes a brake instructing/operating section. The stroke amount of the brake pedal 34 is detected by a brake stroke sensor 35, which constitutes a brake operation amount sensor. The brake stroke sensor 35 can be either a sensor that measures the actual stroke amount of the brake pedal 34 or a master cylinder pressure sensor that senses master cylinder pressure which is indicative of the stroke amount of the brake pedal 34. In either case, the brake stroke sensor 35 outputs the brake stroke amount it detects to a brake controller 36 and the 4WD controller 8. The brake stroke sensor 35 can also be configured and arranged to include a brake switch that indicates that the brake pedal 34 has been depressed by a prescribed amount or not. Thus, a brake on signal is sent to the 4WD controller 8 which is indicative of the brake pedal 34 having been depressed by the prescribed amount.

The brake controller 36 controls the braking force acting on the vehicle by controlling the braking devices (e.g., disc brakes) 37FL, 37FR, 37RL and 37RR installed on the wheels 1L, 1R, 3L and 3R in response to the inputted brake stroke amount by the brake pedal 34. In other words, based on the brake pedal depression amount received from the brake stroke sensor 35, the brake controller 36 controls the brake force applied to the wheels 1L, 1R, 3L and 3R by the brake devices 37FL, 37FR, 37RL and 37RR, which are disc brakes or the like. Each of the braking devices 37FL, 37FR, 37RL and 37RR includes a wheel cylinder pressure sensor that is operatively connected to the 4WD controller 8.

As shown in FIG. 2, the generator 7 is equipped with a voltage adjuster 22 (regulator) for adjusting the output voltage V. The 4WD controller 8 controls the generator load torque Th against the internal combustion engine 2 and the generated voltage V by adjusting the field current Ifh such as controlling a generator control command value c1 (duty ratio or field current value). In other words, for example, based on the generator control command value c1 (duty ratio) issued from the 4WD controller 8, the voltage regulator 22 controls the field current Ifh of the electric generator 7 and thereby controls the generator load torque Th imposed on the engine 2 by the generator 7 and the output voltage V generated by the generator 7. In short, the voltage adjuster 22 receives the generator control command value c1 (duty ratio or field current value) from the 4WD controller 8 and adjusts the field current Ifh of the generator 7 to a value corresponding to the generator control command value c1. The voltage adjuster 22 is also configured and arranged to detect the output voltage V of the generator 7 and then output the detected voltage value to the 4WD controller 8. This arrangment constitutes the generator output voltage regulating section.

A current sensor 23 is provided inside the junction box 10. The current sensor 23 detects the current value Ia of the electrical power supplied from the generator 7 to the electric motor 4 and outputs a detected armature current signal to the 4WD controller 8. The voltage value flowing through the electrical line 9 is detected by the 4WD controller 8 to produce a control signal indicative of the voltage across the electric motor 4. A relay 24 shuts off or connects the voltage (current) supplied to the electric motor 4 in accordance with a control command from the 4WD controller 8.

A control command from the 4WD controller 8 controls the field current Ifm of the electric motor 4 to adjust the drive torque of the electric motor 4 to a target motor torque Tm. In other words, the adjustment of the field current Ifm by the 4WD controller 8 adjusts the drive torque of the electric motor 4 to the target motor torque Tm. A thermistor 25 measures the brush temperature of the electric motor 4 and produces a control signal indicative of the temperature of the electric motor 4 that is outputted to the 4WD controller 8.

The vehicle driving force control apparatus is also equipped with a motor rotational speed sensor 26 that detects the rotational speed Nm of the drive shaft of the electric motor 4. The motor rotational speed sensor 26 outputs a control signal indicative of the detected rotational speed of the electric motor 4 to the 4WD controller 8. The motor rotational speed sensor 26 constitutes an input shaft rotational speed detector or sensor of the clutch 12.

The wheels 1L, 1R, 3L and 3R are provided with wheel speed sensors 27FL, 27FR, 27RL, and 27RR, respectively. Each speed sensor 27FL, 27FR, 27RL, and 27RR outputs a pulse signal corresponding to the rotational speed of the respective wheel 1L, 1R, 3L and 3R to the 4WD controller 8. Each of the pulse signals serves as a wheel speed detection value indicative of the rotational speed of the respective wheel 1L, 1R, 3L and 3R, respectively. The wheel speed sensors 27RL and 27RR constitute an output shaft rotational speed detector or sensor of the clutch 12.

As shown in FIG. 3, the 4WD controller 8 is equipped with a generator control unit or section 8A, a relay control unit or section 8B, a motor control unit or section 8C, a clutch control unit or section 8D, a surplus torque computing unit or section 8E, a target torque limiting unit or section 8F, a surplus torque converting unit or section 8G, a four-wheel drive termination processing unit or section 8H and a TCS target revising unit or section 8J.

The 4WD controller 8 is a control unit that preferably includes a microcomputer with a 4WD control program that is operatively coupled to the internal combustion engine 2 and the electric motor 4 to control the torque applied to the left and right front wheels 1L and 1R by the internal combustion engine 2 and the torque applied to the left and right rear wheels 3L and 3R by an electric motor 4 as discussed below. The 4WD controller 8 can also include other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. The memory circuit stores processing results and control programs. The RAM of the 4WD controller 8 stores statuses of operational flags and various control data for the control program. The ROM of the 4WD controller 8 stores various operations for the control program. The 4WD controller 8 is capable of selectively controlling any of the components of the driving force control apparatus in accordance with the control program. It will be apparent to those skilled in the art from this disclosure that the precise structure and algorithms for 4WD controller 8 can be any combination of hardware and software that will carry out the functions of the present invention. In other words, “means plus function” clauses as utilized in the claims should include any structure including, but not limited to, hardware and/or algorithm or software that can be utilized to carry out the function of the “means plus function” clause. Moreover, the terms “device” and “section” as utilized in the claims should include any structure, i.e., hardware alone, software alone, or combination of hardware and software.

The generator control section 8A uses the voltage regulator 22 to monitor the output voltage of the electric generator 7. Thus, the generator control section 8A is configured to output the generator control command value c1 of the generator 7 to adjust the field current Ifh in accordance with the generator command value c1. In other words, the generator control section 8A adjusts the field current Ifh of the electric generator 7 in such a manner as to obtain a prescribed output voltage V.

The relay control section 8B controls (connection and disconnection) shutting off and connecting the electric power supplied from the generator 7 to the electric motor 4. In short, it functions to switch the motor 4 between a driven state and a non-driven state.

The monitor control section 8C adjusts the torque of the electric motor 4 to the required prescribed value by adjusting the field current Ifm of the electric motor 4.

The clutch control section 8D controls the state of the clutch 12 by outputting a clutch control command to the clutch 12. In other words, the clutch control section 8D operates the clutch 12 to an engaged (connected) state when the vehicle is determined to be in a four-wheel drive mode.

Based on the input signals, the surplus torque computing section 8E, the target torque limiting section 8F, and the surplus torque converting section 8G execute their respective processing sequences in series (i.e., first 8E, then 8F, then 8G, back to 8E, etc.) in accordance with a prescribed sampling time.

The processing sequence executed by the surplus torque computing section 8E will now be described with reference to FIG. 4.

First, in step S10, the surplus torque computing section 8E computes the slippage speed or velocity ΔV_(F), which is the magnitude of the acceleration slippage of the front wheels 1L and 1R. In particular, the average wheel speeds are computed based on the signals from the wheel speed sensors 27FL, 27FR, 27RL and 27RR. The surplus torque computing section 8E subtracts the average wheel speed of the rear wheels 3L and 3R (subordinate drive wheels) from the wheel speed of the front wheels 1L and 1R (main drive wheels) to find the slippage speed or velocity ΔV_(F).

An example of how the slippage velocity ΔV_(F) can be computed will now be presented.

First, the average front wheel speed V_(Wf) (which is the average of the left and right wheel speeds for the front wheels 1L and 1R) and the average rear wheel speed V_(Wr) (which is the average of the left and right wheel speeds for the rear wheels 3L and 3R) are calculated using the following two Equations (1) and (2): V _(Wf)=(V _(Wfl) +V _(Wfr))/2  (1) V _(Wr)=(V _(Wrl) +V _(Wrr))/2  (2)

Second, the slippage speed (acceleration slippage magnitude) ΔV_(F) of the front or main drive wheels 1L and 1R is calculated by the differential between the average front wheel speed V_(Wf) and the average rear wheel speed V_(Wr), using the following Equation (3): ΔV _(F) =V _(Wf) −V _(Wr)  (3)

Then, the 4WD controller 8 proceeds to step S20.

In step S20, the surplus torque computing section 8E of the 4WD controller 8 determines whether or not the calculated slippage velocity ΔV_(F) exceeds a prescribed value, such as zero. Thus, steps S10 and S20 constitute an acceleration slippage detection section that estimates if acceleration slippage is occurring in the front wheels 1L and 1R that is driven by the internal combustion engine 2. If slippage velocity ΔV_(F) is determined to be zero or below, it is estimated that the front wheels 1L and 1R are not experiencing acceleration slippage and the 4WD controller 8 proceeds to step S30, where a target generator load torque Th is set to zero and the surplus torque computing section 8E of the 4WD controller 8 returns to the beginning of the control loop and the 4WD controller 8 returns to the main program.

Conversely, in step S20 if the slippage velocity ΔV_(F) is determined to be larger than zero, then the surplus torque computing section 8E estimates that the front wheels 1L and 1R are experiencing acceleration slippage, and thus, control proceeds to step S40.

In step S40, the surplus torque computing section 8E computes the amount of torque TΔV_(F) that must be absorbed in order to suppress the acceleration slippage of the front wheels 1L and 1R. In other words, the absorption torque TΔV_(F) is an amount that is proportional to the acceleration slippage magnitude. The absorption torque TΔV_(F) is calculated using the following Equation (4): TΔV _(F) =K1×ΔV _(F)  (4) where: K1 is a gain that is found through experimentation or the like.

Then, the surplus torque computing section 8E of the 4WD controller 8 proceeds to step S50.

In step S50, a current load torque TG of the generator 7 is calculated by the surplus torque computing section 8E based on the Equation (5) below, and then the surplus torque computing section 8E proceeds to step S60.

$\begin{matrix} {{TG} = {{K2}\frac{V \times {Ia}}{{K3} \times {Nh}}}} & (5) \end{matrix}$ where: V: voltage of the generator 7,

-   -   Ia: armature current of the generator 7,     -   Nh: rotational speed of the generator 7,     -   K3: efficiency, and     -   K2: coefficient.

In step S60, the surplus torque computing section 8E computes surplus torque, i.e., the target generator load torque Th to be imposed by the electric generator 7. For example, the target generator load torque Th is found based on the Equation (6) stated below, and the 4WD controller 8 returns to the beginning of the control loop. Th=TG+TΔV _(F)  (6)

Next, the processing executed by the target torque (control) limiting section 8F will be explained based on FIG. 5. The processing of the target generator load torque Th in the flow chart of FIG. 5 constitutes a generator control section configured to control a generation load torque of the generator 7 to substantially correspond to an acceleration slippage magnitude of the drive wheel, when the acceleration slippage detection section estimates acceleration slippage occurring in the drive wheel.

First, in step S110, the target torque limiting section 8F of the 4WD controller 8 determines whether or not the target generator load torque Th is larger than the maximum load capacity HQ of the generator 7. The target torque limiting section 8F proceeds to the beginning of the control program to repeat the processing if the target torque limiting section 8F determines that target generator load torque Th is less than or equal to the maximum load capacity HQ of the generator 7. Conversely, the target torque limiting section 8F proceeds to step S120 if the 4WD controller 8 determines that the target generator load torque Th is larger than the maximum load capacity HQ of the generator 7.

In step S120, the target torque limiting section 8F calculates the excess torque ΔTb, which is the amount by which the target generator load torque Th exceeds the maximum load torque HQ. The excess torque ΔTb can be calculated according to the following Equation (7): ΔTb=Th−HQ.  (7) Then, the target torque limiting section 8F proceeds to step S130.

In step S130, the target torque limiting section 8F calculates the current engine torque Te. For example, the current engine torque Te is computed based on the signals from the throttle sensor 19 a and the engine rotational speed sensor 21 using an engine torque calculation map. Then, the 4WD controller 8 proceeds to step S140.

In step S140, the target torque limiting section 8F computes the engine torque upper limit value TeM. The engine torque upper limit value TeM is calculated by subtracting the excess torque ΔTb from the engine torque Te, as set forth in the following Equation (8): TeM=Te−ΔTb.  (8) After the engine torque upper limit value TeM is outputted to the engine controller 18, the target torque limiting section 8F proceeds to step S150.

In step S150, the target torque limiting section 8F substitutes the maximum load capacity HQ as the target generator load torque Th. In other words, the maximum load capacity HQ is assigned as the target generation load torque Th, and then the target torque limiting section 8F returns to the beginning of the control loop and the 4WD controller 8 returns to the main program.

Next, the processing executed by the surplus torque converting section 8G will be explained based on FIG. 6.

First, in step S200, the surplus torque converting section 8G of the 4WD controller 8 determines if the target generator load torque Th is larger than 0. If the target generator load torque Th is determined to be larger than 0, then the program of the surplus torque converting section 8G proceeds to step S210 because the front wheels 1L and 1R are experiencing acceleration slippage. If the surplus torque converting section 8G determines that the target generator load torque Th is less than or equal to 0, then the surplus torque converting section 8G returns to the beginning of the control loop because the front wheels 1L and 1R are not experiencing acceleration slippage. Thus, the vehicle remains in a two-wheel drive mode.

In step S210, the surplus torque converting section 8G determines if the vehicle will shift from four-wheel drive state or mode to two-wheel drive state or mode. If the surplus torque converting section 8G determines that the vehicle will shift to two-wheel drive, then the surplus torque converting section 8G proceeds to step S270 where it executes a four-wheel drive termination processing such as releasing the clutch 12 and stopping the electric generator 7 (c1=0) before returning to the beginning of the control sequence. The surplus torque converting section 8G determines that the vehicle will shift to two-wheel drive in such cases as when the rotational speed of the electric motor 4 has approached an allowable rotational speed limit or when the target motor torque Tm is on a decreasing trend and is found to have become equal to or less than a prescribed threshold value (e.g., 1 N-m) or when the transmission 5 a is in a non-drive range (such as Park or Neutral). Meanwhile, the surplus torque converting section 8G proceeds to step S220 for regular processing if no transition to two wheels is being made and the vehicle remains in the four-wheel drive state or mode.

In step S220, the surplus torque converting section 8G reads in the rotational speed Nm of the electric motor 4 detected by the motor rotational speed sensor 21. The target motor field current Ifmt corresponding to the rotational speed Nm of the electric motor 4 is calculated. Then the surplus torque converting section 8G sends the calculated target motor field current Ifmt to the motor control section 8C before the surplus torque converting section 8G proceeds to step S230.

The target motor field current Ifmt is held at a fixed current value with respect to the rotational speed Nm of the electric motor 4 so long as the rotational speed Nm is less than a prescribed rotational speed. When the rotational speed Nm exceeds the prescribed rotational speed, the field current Ifm of the electric motor 4 is reduced using a well-known weak field control method. More specifically, when the rotational speed of the electric motor 4 becomes high, the motor induced voltage E increases and the motor torque declines. Thus, when the rotational speed Nm of the electric motor 4 exceeds a prescribed value, the field current Ifmt of the electric motor 4 is reduced in order to reduce the induced voltage E and thereby increase the current flowing to the motor 4 in order to obtained the required motor torque. As a result, even if the rotational speed of the electric motor 4 becomes high, the required torque can be obtained because the induced voltage E is kept from increasing so that the motor torque is kept from declining. Since the motor field current Ifmt is controlled in two stages, i.e., one field current is used for rotational speeds below a prescribed rotational speed and another field current is used for rotational speeds equal to or above a prescribed rotational speed, the cost of the electronic circuitry can be reduced in comparison with a case in which the field current is controlled on a continuous basis.

It is also acceptable to provide a motor torque correcting section that corrects the motor torque on a continuous basis by adjusting the field current Ifmt in accordance with the rotational speed Nm of the electric motor 4. In other words, it is acceptable to adjust the field current Ifmt of the electric motor 4 on a continuous basis in accordance with the motor rotational speed Nm instead of on a two-stage basis. Here again, even if the rotational speed of the electric motor 4 becomes high, the required torque can be obtained because the induced voltage E of the electric motor 4 is kept from increasing so that the motor torque is kept from declining. This approach provides a smooth motor torque characteristic and thereby enables the vehicle to travel in a more stable manner than in the case of two-stage control and the motor to be driven in an efficient state at all times.

In step S230, the surplus torque converting section 8G calculates the target motor torque Tm (n) corresponding to the generator load torque Th calculated by the surplus torque computing section 8E using a map or the like and proceeds to step S240.

The step S240, the surplus torque converting section 8G starts up the TCS target revising section and proceeds to step S250. The target motor torque Tm (n) is the target motor output.

In step S250, the surplus torque converting section 8G calculates the target armature current Ia corresponding to the target motor torque Tm (n) and the target motor field current Ifm using a map or the like and proceeds to step S260.

In step S260, the surplus torque converting section 8G computes the duty ratio c1 (i.e., the generator control value) required to achieve the target motor voltage based on the target armature current Ia and outputs the duty ratio c1 before ending the control sequence.

The processing sequence executed by the TCS target revising section 8J will now be described with reference to FIG. 7. The TCS target revising section 8J constitutes an acceleration slippage degree revising section.

First, in step S310, the TCS target revising section 8J finds the target engine rotational speed with which the generator output voltage required to achieve the target motor torque Tm can be obtained. For example, as shown in FIG. 8, a map or the like plotting the target engine rotational speed versus the vehicle speed (which is proportional to the rotational speed of the electric motor) for various target motor torques can be prepared in advance and the target engine rotational speed can be found based on the current vehicle speed and the target motor torque Tm. FIG. 8 is a simplified example. In other words, the lines for each of the target motor torques Tm are not necessarily straight.

In step S320, the TCS target revising section 8J calculate the difference ΔNe between the target engine rotational speed found in the previous step S310 and the current engine rotational speed (i.e., ΔNe=target engine rotational speed−current engine rotational speed) and determines if the rotational speed difference ANe is equal to or greater than a prescribed threshold value ΔNe1 whose value is a non-negative number (≧0). If the rotational speed difference ΔNe is equal to or greater than the threshold value ΔNe1, the TCS target revising section 8J proceeds to step S330. If the rotational speed difference ΔNe is less than the threshold value ΔNe1 (e.g., if the ΔNe is a negative value), the TCS target revising section 8J proceeds to step S340.

In step S330, the TCS target revising section 8J computes the target slippage ratio revision amount ΔTslip corresponding to the rotational speed difference ΔNe using information expressing the relationship between the rotational speed difference ΔNe and the target slippage ratio revision amount ΔTslip, such as the plot shown in FIG. 9. Then, the TCS target revising section 8J increases the target slippage ratio Tslip by adding the target slippage ratio revision amount ΔTslip to a reference target slippage ratio Tslip0 (e.g., 10%) as set forth in the following Equation (9): Tslip=Tslip0+ΔTslip  (9)

The reference target slippage ratio Tslip0 is normally equal to a default value, but in some cases it can be modified by another control operation. In such cases, the modified value of Tslip0 is used in step S330.

FIG. 9 is a simplified diagram depicting a first-degree proportional relationship between the rotational speed difference ΔNe and the target slippage ratio revision amount ΔTslip, but the invention is not limited to such a relationship. It is also acceptable to prepare information expressing the relationship between the rotational speed difference ΔNe and the target slippage ratio for each gear ratio of the transmission 5 a as shown in FIG. 10. The reference target slippage ratio Tslip0 is set in advance to such a value as 10%.

Next, the processing executed by the engine controller 18 will be described with reference to FIG. 11. In accordance with a prescribed sampling time (i.e., once per sampling time period), the engine controller 18 executes the processing show in FIG. 11 based on the various input signals.

In step S590, the engine controller 18 finds the acceleration slippage ratio ΔV of the front wheels and proceeds to step S600. In step S600, the engine controller 18 determines if the acceleration slippage ratio ΔV exceeds the target slippage ratio Tslip. If the acceleration slippage ratio ΔV does exceed the target slippage ratio Tslip, then the engine controller 18 proceeds to step 700. If the acceleration slippage ratio ΔV is equal to or less than the target slippage ratio Tslip, then the engine controller 18 proceeds to step S610. Also, as explained previously, the target slippage ratio Tslip is revised to a value larger than the reference value when there is the possibility that the electric generator 7 will not be able to generate an output voltage sufficient to achieve the target motor torque Tm.

In step S610, the engine controller 18 computes the target engine output torque TeN requested by the driver based on the detection signal received from the accelerator sensor 29 and proceeds to step S620.

In step S620, the engine controller 18 determines if the engine output torque limit TeM is being received from the 4WD controller 8. If so, the engine controller 18 proceeds to step S630. If not, the engine controller proceeds to step S670.

In step S630, the engine controller 18 determines if the engine output torque limit TeM is smaller than the target engine output torque TeN. If the engine output torque limit TeM is smaller, the engine controller 18 proceeds to step S640. Meanwhile, if the engine output torque limit TeM is equal to or larger than the target engine output torque TeN, the engine controller 18 proceeds to step S670.

In step S640, the engine controller 18 limits the target engine output torque TeN by substituting the engine output torque limit TeM for the target engine output torque TeN and proceeds to step S670.

In step S670, the engine controller 18 calculates the current engine output torque Te based on the throttle opening, engine rotational speed, etc., and proceeds to step S680.

In step S680, the engine controller 18 finds the difference ΔTe′ between the current output torque and the target output torque TeN. The difference ΔTe′ is calculated using the following Equation (10): ΔTe′=TeN−Te  (10)

Then, the engine controller 18 proceeds to step S690.

Meanwhile, in step S700, the engine controller 18 executes so-called TCS control to substitute a prescribed TCS torque reduction quantity (>0) for the difference ΔTe′. This step constitutes the main drive source output suppressing section.

In step S690, the engine controller 18 calculates an adjustment amount Δθ of the throttle opening θ corresponding to the difference ΔTe′ and sends a throttle opening signal corresponding to the throttle opening adjustment amount Δθ to the stepping motor 19 before returning to the beginning of the control sequence.

In the preceding explanation, the engine controller 18 is described as issuing a throttle opening signal Δθ corresponding to the difference ΔTe′ in order to simplify the explanation. In actual practice, however, the throttle opening θ is adjusted in successive increments corresponding to a prescribed torque increase amount or torque decrease amount each time step S700 is executed so that the torque and other operating conditions will change smoothly.

The operational effects of the device described heretofore will now be described.

When the torque transmitted to the front wheels 1L and 1R from the engine 2 exceeds the road surface reaction force torque limit, i.e., when the front wheels 1L and 1R (main drive 1L and 1R) undergo acceleration slippage, due to the coefficient of friction μ of the road surface being small or the accelerator pedal 17 being depressed deeply by the driver, the clutch 12 is connected and the electric generator 7 is operated with a load torque Th corresponding to the magnitude of the acceleration slippage, thus shifting the vehicle into four-wheel drive mode. The surplus electric power generated by the electric generator 7 is then used to drive the electric motor 4, which in turn drives the subordinate drive wheels, i.e., the rear wheels 3L and 3R. This driving of the subordinate wheels improves the acceleration performance of the vehicle. Also, since the electric motor 4 is driven using the surplus torque in excess of the road surface reaction torque limit of the main drive wheels 1L and 1R, the energy efficiency and fuel economy of the vehicle are also improved.

Afterwards, the drive torque transmitted to the front wheels 1L and 1R is adjusted so as to approach a value close to the road surface reaction torque limit of the front wheels 1L and 1R and the vehicle is shifted back to two-wheel drive. As a result, acceleration slippage of the front wheels 1L and 1R (main drive wheels) is suppressed.

Meanwhile, if the acceleration slippage ratio exceeds the target slippage ratio Tslip, engine TCS control is executed to drop the engine torque and suppress the acceleration slippage of the main drive wheels, i.e., the front wheels 1L and 1R, separately from the operation of the electric generator 7.

As the vehicle speed increases and the transmission 5 a shifts to second speed and above, the rotational speed of the engine 2 decreases in comparison with first speed. For example, when the transmission upshifts from first speed to second speed, the engine rotational speed decreases and the output voltage of the electric generator 7 decreases, posing the possibility that it will become impossible to achieve the target motor torque Tm. Furthermore, when a prescribed target slippage ratio Tslip is exceeded, the engine TCS control is executed to suppress the engine output and thereby suppress the acceleration slippage, but this causes the vehicle to return to two-wheel drive. In short, there is the possibility that the operating region in which four-wheel drive is used will essentially be limited to first speed.

However, with the present invention, when the vehicle is in a four-wheel drive state with the electric motor 4 being driven by the electric generator 7, the TCS target revising section 8J finds a generator rotational speed, i.e., target engine rotational speed, with which a generator output voltage corresponding to the target motor torque Tm (n) can be obtained. If the target engine rotational speed is larger than the actual engine rotational speed by a prescribed threshold value, then it determines that a generator output voltage corresponding to the target motor torque cannot be obtained and executes engine TCS control to increase the target slippage ratio Tslip and thereby increase the amount of acceleration slippage of the front wheels.

Consequently, with the present invention, even if the rotational speed of the engine 2 decreases when, for example, the transmission 5 a upshifts from first speed to second speed, the amount of acceleration slippage allowed at the main drive wheels, i.e., the front wheels, increases. Thus, even when the transmission 5 a is in second speed or higher, the engine rotational speed and the generator rotational speed increase due to the increased acceleration slippage. As a result, the operating region in which the electric motor 4 can deliver the target motor torque, i.e., the operating region in which the vehicle can perform four-wheel drive without incurring speed losses, is expanded.

Although the first embodiment presents a case in which the vehicle shifts to four-wheel drive in response to the occurrence of acceleration slippage at the front wheels 1L, 1R, the present invention can also be applied to systems that shift to four-wheel drive when the vehicle accelerates from a stopped condition or when the accelerator pedal is depressed to a certain degree. In short, the conditions under which the electric motor 4 is driven are not limited to those described above.

Although in the first embodiment, the engine TCS control is executed based on the target slippage ratio Tslip, it is also acceptable to execute the control based on a target slippage amount. In other words, the target slippage degree is not limited to the slippage ratio and can be expressed as another quantity, such as the slippage amount.

It is also acceptable to design the drive force control apparatus such that if the actual acceleration slippage ratio does increase toward the target slippage ratio after the target slippage ratio has been increased, the engine rotational speed is increased irregardless of the accelerator position so that the slippage amount is increased with certainty.

Second Embodiment

Referring now to FIG. 12, a vehicle driving force control apparatus in accordance with a second embodiment will now be explained. The configuration of the vehicle in this second embodiment is the same as the configuration of the vehicle in the first embodiment (see FIG. 1). In view of the similarity between the first and second embodiments, the parts or steps of the second embodiment that are identical to the parts or steps of the first embodiment will be given the same reference numerals as the parts or steps of the first embodiment. Moreover, the descriptions of the parts or steps of the second embodiment that are identical to the parts or steps of the first embodiment may be omitted for the sake of brevity. In other words, unless otherwise specified, the rest of the configuration of the vehicle and the processing in the second embodiment are the same as the configuration of the first embodiment. More specifically, the constituent features of the second embodiment are basically the same as those of the first embodiment, except that the processing executed by the TCS target revising section 8J when the electric motor 4 is being driven is different. Otherwise, the second embodiment is the same as the first embodiment.

The processing executed by the TCS target revising section 8J of the second embodiment is shown in FIG. 12. First, in step S800, the TCS target revising section 8J determines if the transmission 5 a is in first speed or second speed. If the TCS target revising section 8J finds that the transmission 5 a is in first speed, then the TCS target revising section 8J proceeds to step S810. If the TCS target revising section 8J finds that the transmission 5 a is in second speed, then the TCS target revising section 8J proceeds to step S820.

In step S810, the TCS target revising section 8J sets the target slippage ratio Tslip to the reference acceleration slippage ratio Tslip0 (e.g., 10%).

In step S820, the TCS target revising section 8J sets the target slippage ratio Tslip to a second speed acceleration slippage ratio Tslip2 (e.g., 20%) that is larger than the reference acceleration target slippage ratio Tslip0.

As the vehicle speed increases and the transmission 5 a shifts from first speed to second speed, there is the possibility that the engine rotational speed will decline and make it impossible to generate the required electric power. However, with this embodiment, the target slippage ratio Tslip used for engine TCS control is increased when the transmission upshifts to second speed such that the allowable acceleration slippage amount is increased in comparison with first speed. As a result, even if the engine rotational speed drops due to the upshift, the amount of acceleration slippage at the main drive wheels (front wheels) increases and curbs the amount by which the engine rotational speed drops. In short, the amount by which the output of the electric generator declines due to the upshift can be held in check and four-wheel drive can be utilized even when the transmission upshifts, i.e., the operating region in which four-wheel drive can be used is expanded.

Although the second embodiment illustrates a case in which four-wheel drive utilizing the electric motor 4 is executed in the operating region encompassing first speed and second speed, it is also acceptable to expand the region to include third speed or above. In such a case, acceleration slippage ratios should be prepared for each gear ratio. It is also acceptable to change the gear ratio used depending on the vehicle speed even when the gear ratio remains the same.

Also, although the second embodiment illustrates a case in which the target slippage ratio Tslip is increased after the transmission 5 a upshifts to second speed, it is also acceptable to increase the target slippage ratio Tslip immediately before the transmission shifts from first speed to second speed.

All other operational effects and constituent features are the same as the first embodiment.

As used herein to describe the above embodiments, the following directional terms “forward, rearward, above, downward, vertical, horizontal, below and transverse” as well as any other similar directional terms refer to those directions of a vehicle equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to a vehicle equipped with the present invention.

The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.

This application claims priority to Japanese Patent Application No. 2003-364314. The entire disclosure of Japanese Patent Application No. 2003-364314 is hereby incorporated herein by reference.

While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments. 

1. A vehicle driving force control apparatus comprising: a main drive source configured and arranged to drive a main drive wheel; an electric generator configured and arranged to be driven by the main drive source; an electric motor configured and arranged to drive a subordinate drive wheel from output voltage supplied from the electric generator; a main drive source output suppressing section configured to suppress the output of the main drive source such that a degree of acceleration slippage of the main drive wheel is matched to a preset target slippage degree when the main drive wheel undergo acceleration slippage; and an acceleration slippage degree revising section configured to increase the target slippage degree upon estimating that at least one of a target generator output voltage will likely be insufficient to achieve a target motor output of the electric motor, and a gear ratio of a transmission has increased.
 2. The vehicle driving force control apparatus as recited in claim 1, wherein the acceleration slippage degree revising section is configured to increase the target slippage degree upon estimating the target generator output voltage will likely be insufficient to achieve the target motor output.
 3. The vehicle driving force control apparatus as recited in claim 2, wherein the acceleration slippage degree revising section is configured to find a target rotational speed of the main drive source required to attain the target motor output and revise the target slippage degree in accordance with a difference between the target rotational speed and an actual rotational speed of the main drive source.
 4. The vehicle driving force control apparatus as recited in claim 3, wherein the acceleration slippage degree revising section being configured to change the amount by which the target slippage degree is revised in accordance with a rotational speed difference depending on the gear ratio of the transmission.
 5. The vehicle driving force control apparatus as recited in claim 2, wherein the acceleration slippage degree revising section being configured to change the amount by which the target slippage degree is revised in accordance with a rotational speed difference depending on the gear ratio of the transmission.
 6. The vehicle driving force control apparatus as recited in claim 2, further comprising a generator output voltage regulating section configured to regulate the output voltage of the electric generator such that the output of the electric motor is matched to the target motor output.
 7. The vehicle driving force control apparatus as recited in claim 2, wherein the electric generator and the electric motor are both driven at least when the main drive wheel is undergoing acceleration slippage.
 8. The vehicle driving force control apparatus as recited in claim 2, wherein the main drive source is an internal combustion engine.
 9. The vehicle driving force control apparatus as recited in claim 1, further comprising a generator output voltage regulating section configured to regulate the output voltage of the electric generator such that the output of the electric motor is matched to the target motor output.
 10. The vehicle driving force control apparatus as recited in claim 1, wherein the electric generator and the electric motor are both driven at least when the main drive wheel is undergoing acceleration slippage.
 11. The vehicle driving force control apparatus as recited in claim 1, wherein the main drive source is an internal combustion engine.
 12. The vehicle driving force control apparatus as recited in claim 1, wherein the acceleration slippage degree revising section is configured to increase the target slippage degree upon estimating the gear ratio of the transmission has increased.
 13. The vehicle driving force control apparatus as recited in claim 12, wherein the electric generator and the electric motor are both driven at least when the main drive wheel is undergoing acceleration slippage.
 14. The vehicle driving force control apparatus as recited in claim 13, wherein the main drive source is an internal combustion engine.
 15. The vehicle driving force control apparatus as recited in claim 12, wherein the main drive source is an internal combustion engine.
 16. The vehicle driving force control apparatus as recited in claim 12, further comprising a generator output voltage regulating section configured to regulate the output voltage of the electric generator such that the output of the electric motor is matched to the target motor output.
 17. A vehicle drive force control apparatus comprising: main drive means for rotating a main drive wheel; generator means for generating electric power by rotation of the drive means; electric motor means for rotating a subordinate drive wheel from output voltage supplied from the generator means; main drive source output suppressing means for suppressing the output of the main drive means such that a degree of acceleration slippage of the main drive wheel is matched to a preset target slippage degree when the main drive wheel undergo acceleration slippage; and acceleration slippage degree revising means for increasing the target slippage degree upon estimating that at least one of a target generator output voltage will likely be insufficient to achieve the target motor output of the electric motor means, and a gear ratio of a transmission has increased.
 18. A method of controlling a vehicle drive force control apparatus comprising: generating electric power using a generator that is driven by rotation of a main drive source used for rotating a main drive wheel; supply electric power to an electric motor from the generator for rotating a subordinate drive wheel from output voltage supplied from the generator; suppressing an output of the main drive main drive source such that a degree of acceleration slippage of the main drive wheel is matched to a preset target slippage degree when the main drive wheel undergo acceleration slippage; and increasing the target slippage degree upon estimating that at least one of a target generator output voltage will likely be insufficient to achieve a target motor output of the electric motor, and a gear ratio of a transmission has increased.
 19. The method as recited in claim 18, wherein the increasing of the target slippage degree is performed by estimating the gear ratio of the transmission has increased.
 20. The method as recited in claim 18, wherein the increasing of the target slippage degree is performed by estimating the target generator output voltage will likely be insufficient to achieve the target motor output. 